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Organometallics 2010, 29, 1312–1322 DOI: 10.1021/om900258q
Surface Organometallic Chemistry of Hf(CH2tBu)4 on Silica and Silica-Alumina: Reaction of the Resulting Grafted Hafnium Neopentyl with Dihydrogen Geraldine Tosin, Marco Delgado, Anne Baudouin, Catherine C. Santini,* Franc-ois Bayard, and Jean-Marie Basset Universit e de Lyon, Institut de Chimie de Lyon, LC2P2, Equipe de Chimie Organom etallique de Surface, UMR 5265 CNRS-ESCPE Lyon, 43 Boulevard du 11 Novembre 1918, F-69626 Villeurbanne Cedex, France Received April 6, 2009
[Hf(CH2tBu)4], 1, has been found to react at room temperature with an aerosil silica dehydroxylated at different temperatures θ (θ = 800, 500, 200 °C) to give a modified surface referred to as 1/SiO2-(θ) and with a silica-alumina to give a modified surface referred to as 1/SiO2-Al2O3-(500). With SiO2-(800) a single-site monosiloxy surface complex [(tSiO)Hf(CH2tBu)3], 1-SiO2-(800), is obtained. Contrarily, with SiO2-(500) a monosiloxy 1-SiO2-(500) and a bis-siloxy surface complex [(tSiO)2Hf(CH2tBu)2], 2-SiO2-(500), are formed in a ratio of 70%:30%. With SiO2-(200), there is mainly the formation of the bis-siloxy surface complex (up to 90%) but in different local environments: (tSiO)2Hf(CH2tBu)2, 2-SiO2-(200), and (tSiO)2(tSiOH)Hf(CH2tBu)2, 20 -SiO2-(200). Finally with SiO2-Al2O3-(500), two major neutral surface complexes are formed: the monosiloxy 1-SiO2-Al2O3-(500) and the bisiloxy 2-SiO2-Al2O3-(500), as well as a third complex, 3, which is not as well-defined and may be cationic. Under hydrogen at 150 °C, 17 h, both modified surface 1/SiO2-(θ) and 1/SiO2-Al2O3-(500) afford the same surface hydrides but in different proportion and diverse surface “local environments”. The formation of these hydrides is concomitant with the formation of [(tSiO)2Si(H)2] and [(tSiO)3SiH]. With 1/SiO2-Al2O3-(500), the formation of [(tSiO)nAlH] is also observed. The major surface hydride in the hydrogenolysis of 1/SiO2-(800), 1/SiO2-(500), and 1/SiO2Al2O3-(500) is (tSiO)2Hf(H)2, whereas in the hydrogenolysis of 1/SiO2-(200), (tSiO)3Hf(H) forms preferentially. All these alkyl and hydride surface complexes have been fully characterized by elemental analysis, labeling experiments, infrared, 1H/13C solid-state NMR, and 1H DQ solid-state NMR.
Introduction Interactions between silica and the homoleptic metal alkyls MR4 were first investigated in the early 1970s, as a preparative route to obtain highly active “single-site” heterogeneous olefin polymerization catalysts.1-3 In the case of silica pretreated above 200 °C, the major grafting pathway was assumed to be the protolysis of one of the M-C bonds by one surface silanol (tSi-OH) to form a single tSiO-M bond. This first approach has been reinvestigated in greater detail using modern tools of surface organometallic chemistry.4 With this new approach, well-defined monosiloxy surface complexes were obtained such as [(tSiO)Zr(CH2tBu)3],5,6 [(tSiO)Ta(dCHt*Corresponding author. E-mail:
[email protected]. (1) Ballard, D. G. H. Adv. Catal. 1973, 23, 263–325. (2) Candlin, J. P.; Thomas, H. Adv. Chem. Ser. 1974, 132, 212–239. (3) Yermakov, Y.; Zakharov., V. Adv. Catal. 1975, 24, 173. (4) Basset, J.-M.; et al. et al. Surface Organometallic Chemistry; Wiley-VCH: Weinheim, 2009. (5) Quignard, F.; Choplin, A.; Basset, J.-M. Chem. Commun. 1991, 1589–1590. (6) Corker, J.; Lefebvre, F.; Lecuyer, C.; Dufaud, V.; Quignard, F.; Choplin, A.; Evans, J.; Basset, J.-M. Science 1996, 271, 966–969. (7) Le Roux, E.; Chabanas, M.; Baudouin, A.; de Mallmann, A.; Coperet, C.; Quadrelli, E. A.; Thivolle-Cazat, J.; Basset, J.-M.; Lukens, W.; Lesage, A.; Emsley, L.; Sunley, G. J. J. Am. Chem. Soc. 2004, 126, 13391–13399. pubs.acs.org/Organometallics
Published on Web 02/24/2010
Bu)(CH2tBu)2], [(tSiO)TaCp*(Me)3], [(tSiO)W(tCtBu)(CH2tBu)2],7-9 and [(tSiO)Hf(CH2tBu)3].10 On silica surfaces, the reaction between alkyl early transition metals and silanol yields only neutral surface complexes. The density and distribution of silanol groups on the partially dehydroxylated silica leads to control of the stoichiometry of a given grafting reaction and the coordination sphere of the metal center. The reaction occurs with the surface silanol (tSi-OH) or siloxane bridge (tSi-O-Sit) moieties, which react as ligands X (oneelectron ligand) (A) or L (two-electron ligand) (B)10-12 and with siloxane bridges with the transfer of an alkyl group to the surface (C) (Scheme 1). The nature of the direct environment around the grafted metal is crucial for the catalytic activity. For instance, during (8) Le Roux, E.; Taoufik, M.; Chabanas, M.; Alcor, D.; Baudouin, A.; Coperet, C.; Thivolle-Cazat, J.; Basset, J.-M.; Lesage, A.; Hediger, S.; Emsley, L. Organometallics 2005, 24, 4274–4279. (9) Le Roux, E.; Taoufik, M.; Coperet, C.; de Mallmann, A.; Thivolle-Cazat, J.; Basset, J.-M.; Maunders, B. M.; Sunley, G. J. Angew. Chem., Int. Ed. 2005, 44, 6755–6758. (10) Tosin, G.; Santini, C. C.; Taoufik, M.; De Mallmann, A.; Basset, J.-M. Organometallics 2006, 25, 3324–3335. (11) Basset, J.-M.; Lefebvre, F.; Santini, C. Coord. Chem. Rev. 1998, 178-180, 1703–1723. (12) Dufour, P.; Houtman, C.; Santini, C. C.; Nedez, C.; Basset, J.M.; Shu, L. Y.; Shore, S. G. J. Am. Chem. Soc. 1992, 114, 4248–4257. r 2010 American Chemical Society
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Scheme 1. Examples of the Coordination Sphere of Supported Transition Metal Complexes on Silica Surfaces
olefin polymerization, the supported cationic zirconium catalyst [(tSiO)ZrCp*(Me)]þ[(Me)B(C6F5)3]- undergoes cleavage of [tSi-O-Sit] bonds, which are present in the vicinity of Zr, with concomitant formation of new tSi-OZr and tSi-Me bonds, affording an inactive bis-siloxy surface complex [(tSiO)2ZrCp*]þ[(Me)B(C6F5)3]-[tSiMe].13 Knowledge of the structure in the first coordination spheres and local environment can provide information on the possible routes of deactivation and consequently some guidelines to improve catalytic activity. The surface alkyl or hydride complexes of group 4 are precursors to several relatively well-defined heterogeneous Ziegler-Natta polymerization and depolymerization catalysts,.13-21 In the case of hafnium, surface complexes are rarely reported.19,22 Recently, we have reported that the reaction of [Hf(CH2tBu)4], 1, with SiO2-(800) led to a singlesite monosiloxy surface complex [(tSiO)Hf(CH2tBu)3], referred to as 1-SiO2-(800).10,21,23 In this paper, we report the full characterization of the surface hafnium complexes formed during the reaction of [Hf(CH2tBu)4] with an aerosil silica dehydroxylated at 500 and 200 °C and with a silica-alumina dehydroxylated at 500 °C, as well as hafnium hydride derived from these grafted alkyls by reaction under hydrogen.
Results and Discussion A. Synthesis of the Neopentylhafnium Surface Complexes through the Reaction of 1 with SiO2-(θ) (θ = 800, 500, 200 °C) and SiO2-Al2O3-(500). Support. Among the large variety of accessible oxide supports, silica is probably the simplest and the best understood. Its surface is composed of relatively unreactive (at least for moderate temperatures of (13) Millot, N.; Soignier, S.; Santini, C. C.; Baudouin, A.; Basset, J.M. J. Am. Chem. Soc. 2006, 128 (29), 9361–9370. (14) Zakharov, V. A.; Dudchenko, V. K.; Paukstis, E.; Karakchiev, L. G.; Ermakov, Y. I. J. Mol. Catal. 1977, 2, 421–35. (15) Zakharov, V. A.; Nesterov, G. A.; Vasnetsov, S. A.; Thiele, K. H. Transition Met. Organomet. Catal. Olefin Polym. [Proc. Int. Symp.] 1988, 91–100. (16) Zakharov, V. A.; Ryndin, Y. A. J. Mol. Catal. 1989, 56, 183–193. (17) Dufaud, V.; Basset, J.-M. Angew. Chem., Int. Ed. 1998, 37, 806– 810. (18) Jezequel, M.; Dufaud, V.; Ruiz-Garcia, M. J.; Carrillo-Hermosilla, F.; Neugebauer, U.; Niccolai, G. P.; Lefebvre, F.; Bayard, F.; Corker, J.; Fiddy, S.; Evans, J.; Broyer, J.-P.; Malinge, J.; Basset, J.-M. J. Am. Chem. Soc. 2001, 123, 3520–3540. (19) Alt, H. G. Dalton Trans. 2005, 20, 3271–3276. (20) Motta, A.; Fragala, I. L.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 16533–16546. (21) Tosin, G.; Santini, C. C.; Basset, J.-M. Top. Catal. 2009. (22) d’Ornelas, L.; Reyes, S.; Quignard, F.; Choplin, A.; Basset, J.-M. Chem. Lett. 1993, 1931–4. (23) Tosin, G.; Santini, C. C.; Baudouin, A.; De Mallmann, A.; Fiddy, S.; Dablemont, C.; Basset, J.-M. Organometallics 2007, 26, 4118–4127.
dehydroxylation) siloxane bridges tSi-O-Sit and surface hydroxyl groups tSi-OH. A huge number of vibrational spectroscopy and solid-state NMR (1H, 29Si) studies have contributed to the determination, the distribution, and the reactivity of these surface reactive sites.24,25 The design of single-site supported organometallic complexes requires the perfect knowledge and the precise control of the surface reactive sites. Their nature and concentration are indeed strongly fundamental in the final structure of the surface organometallic fragments. A preliminary study of the surface morphology as a function of the pretreatment temperature is then essential prior to chemical modification. Silica. Compacted flame Aerosil silica from Degussa was dehydroxylated under vacuum (10-5 Torr) at 800 °C, SiO2-(800) at 500 °C, SiO2-(500) at 200 °C, and SiO2-(200). The specific surface area determined by BET experiments was 200 m2/g for SiO2-(200, 500) and 180 m2/g for SiO2-(800). The IR spectra show there are no H-bonded silanols above 500 °C, and only isolated and terminal silanols at 3747 and 3720 cm-1 were still observed at this temperature. For dehydroxylation at 800 °C, the low-wavenumber asymmetry of the residual isolated silanol peak at 3747 cm-1 disappeared and the band became narrower and more symmetrical. Solid-state 1H NMR spectra of silica samples dehydroxylated at temperatures below 500 °C displayed two peaks at 2.7 and 1.8 ppm. These peaks were respectively assigned to H-bonded and isolated/geminal surface silanols. For dehydroxylation temperatures above 500 °C, the persistence of the sharp peak at 1.8 ppm demonstrated that only isolated silanols were present on the surface.26,27 The influence of the dehydroxylation temperature on the surface concentration ROH of hydroxyl groups (OH/nm2) of the reactive surface sites was studied by infrared and 1H solidstate NMR spectroscopy and found equal to 0.6 ( 0.1, 1.4 ( 0.1, and 3.5 ( 0.1 OH nm-2, respectively.28 Silica-Alumina. The silica-alumina HA-S-HPV from Azko-Nobel, SiO2-Al2O3-(500), is composed of 75% silica and 25% alumina. Silica-alumina was calcined at 500 °C under dry air, then dehydroxylated at 500 °C under dynamic vacuum (10-5 mmHg). After this treatment, the presence of only a sharp peak at 1.7 ppm and a shoulder at 2.5 ppm in the 1 H solid-state NMR spectra and of a narrow and symmetrical (24) Morrow, B. A. Stud. Surf. Sci. Catal. 1990, 57, A161–A224. (25) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry; Wiley: New York, 1979. (26) Bronnimann, C. E.; Zeigler, R. C.; Maciel, G. E. J. Am. Chem. Soc. 1988, 110, 2023–6. (27) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C., Eds. Characterization and Chemical Modification of the Silica Surface; Studies in Surface Science and Catalysis, Vol. 93; Elsevier: Amsterdam, 1995. (28) Millot, N.; Santini, C. C.; Lefebvre, F.; Basset, J.-M. C. R. Chim. 2004, 7, 725–736.
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Table 1. Analytical Data of the Grafting of Hf(CH2tBu)4, 1, with Silica Partially Dehydroxylated at θ SiO2-(θ) (θ = 800, 500, 200 °C) [1/SiO2] and with SiO2-Al2O3-(500) [1/SiO2-Al2O3-(500)]a
HfNp4 mg (μmol)
θ °C pretreatment specific surf (m2 g-1) [OH nm-2]
SiO2 mg [amount (tSiOH) (μmol)b]
NpH evolved/ Hfg ((0.1)
Hfg wt % ((0.2)
Hfg/ nm2
Hfg/Si-OH ((0.1)
C wt % ((0.2)
no. of moles of NpH mol ratio evolved/Hfg after hydrogenolysis C/Hfg ((2) at 150 °C ((0.2)
Silica 166 (360) 135 (290) 268 (580)
800 (180) [0.6 ( 0.1] 500 (200) [1.4 ( 0.1] 200 (200) [3.5 ( 0.1]
1430 [256]
1.0
3.5
0.66
1.1
3.2
14
3.0
500 [290]
1.3
5.0
0.8
0.6
4.1
12
2.8
1000 [1160]
1.8
6.2
1.0
0.3
4.2
10
1.4
0.41
0.5
6.7
12
2.8
Silica-Alumina 309 (670)
500 (390) [1.4 ( 0.1]
590 [540]
1.4
8.0
a Grafted Hf = Hfg. b Determined from the following equation: ROH = (δOHNA)/S. ROH: surface concentration (OH/nm2), δOH: concentration of hydroxyl groups (mmol/g), S: specific area (m2/g), NA: Avogadro number.
Figure 1. IR spectra: (A) (i) SiO2-(800), (ii) SiO2-(800) after sublimation of 1 followed by vacuum treatment (10-5 Torr) at 70 °C, 1 h. (B) (i) SiO2-(500), (ii) SiO2-(500) after sublimation of 1 followed by vacuum treatment (10-5 Torr) at 70 °C, 1 h. (C) (i) SiO2-(200), (ii) SiO2-(200) after sublimation of 1 followed by vacuum treatment (10-5 Torr) at 70 °C, 1 h. (D) (i) SiO2-Al2O3-(500), (ii) SiO2-Al2O3-(500) after sublimation of 1 at 70 °C followed by vacuum treatment (10-5 Torr) at 70 °C 1 h.
ν(O-H) band at 3745 cm-1 in the IR spectra demonstrated that only isolated surface silanols are present. In agreement with other studies, no ν(Al-OH) vibrations have been observed. This can be easily explained, as during the synthesis and/or the dehydroxylation of silica-alumina basic tAl-OH groups would selectively react with acidic tSi-OH groups by (29) Peri, J. B. J. Catal. 1976, 41, 227–239.
condensation to tAl-O-Sit with formation of water.29-31 The density of surface hydroxyl groups tSi-OH is found equal to 1.4 OH nm-2, and the specific surface determined by BET experiments is found to be equal to 390 m2 g-1. (30) Bronnimann, C. E.; Chuang, I. S.; Hawkins, B. L.; Maciel, G. E. J. Am. Chem. Soc. 1987, 109, 1562–4. (31) Baltusis, L.; Frye, J. S.; Maciel, G. E. J. Am. Chem. Soc. 1987, 109, 40–6.
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Figure 2. (i) 1H MAS NMR spectrum of Hf(CH2tBu)4/SiO2-(200). (ii) 13C CP-MAS solid-state NMR spectrum of Hf(CH2tBu)4/ SiO2-(200). (iii) 13C CP-MAS solid-state NMR spectrum of enriched surface complex Hf(13CH2tBu)4/SiO2-(200) 10% randomly enriched on the R-position. (iv) 13C CP-MAS solid-state NMR spectrum of enriched surface complex Hf(13CH2tBu)4/SiO2-(200) after treatment under dry oxygen atmosphere, at room temperature.
Grafting Reaction. The reaction of 1 with the oxide surface was achieved at 25 °C. The evolution of neopentane as the only gaseous product was quantified by gas chromatography reported as NpH evolved/grafted Hf in Table 1. The elemental analysis of the resulting solid determines wt % Hf and wt % C, allowing us to calculate the molar % Hf grafted per nm2 of the support and the ratio C/Hf. Finally hydrogenolysis of the species at 150 °C for 17 h led to the formation of methane (9 mol) and of ethane (3 mol), allowing us to determine the number of moles of neopentyl ligand per mol of grafted hafnium. The sum of all carbon obtained by these different analyses allows us to determine the steochiometry of the reaction of 1 with the surface oxide. In Table 1, the analytical data for the reaction of 1 with SiO2-(500), SiO2-(200), and SiO2-Al2O3-(500) have been collected. By comparison with the data obtained for 1SiO2-(800),10 it appears clearly that the reactions of 1 with SiO2-(500), SiO2-(200), and SiO2-Al2O3-(500) lead to several surface species referred to as 1/SiO2 and 1/SiO2-Al2O3-(500). Infrared Study. Upon grafting of 1 onto SiO2-(θ) (θ = 800, 500, 200 °C) and onto SiO2-Al2O3-(500), there was total
disappearance of the ν(O-H) band at 3747 cm-1, attributed to isolated silanol groups, and the concomitant emergence of bands at 2955 (νas(CH3)), 2867 (νs(CH3)), 1466 (νas(CH3)), and 1364 (νs(CH3)) characteristic of the neopentyl ligands. This observation along with the fact that neopentane was evolved during grafting means that the grafting reaction occurred via protonolysis of the Hf-CH2tBu bond by surface silanols. With SiO2-(500), SiO2-(200), and SiO2Al2O3-(500), the original broad band between 3740 and 3550 cm-1 attributed to interacting vicinal silanol groups was still present (Figure 1). When 1 was sublimed onto a SiO2-Al2O3-(500) disk, the silanol band at 3747 cm-1 disappeared, with the simultaneous appearance of a group of bands in the spectral regions corresponding to the CH vibrations at 2955 (νas(CH3)), 2867 (νs(CH3)), 1466 (νas(CH3)), and 1364 cm-1 (νs(CH3)). The main reaction on the surface seems to take place between the molecular hafnium complex and the surface silanols. The 1H MAS NMR spectra of all samples of Hf(CH2tBu)4/ SiO2-(θ) (θ = 800, 500, 200 °C), referred to as 1/SiO2-(θ), and Hf(CH2tBu)4/SiO2Al2O3-(500), referred to as 1/SiO2Al2O3-(500), show one broad peak centered at 0.8 (Figure 2i) and 0.9 ppm
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Figure 3. Molecular modeling of (A) [(tSiO)Hf(CH2tBu)3] and (B) [(tSiO)2Hf(CH2tBu)2]. The green surface corresponds to accessible surface (Connolly surfaces, Experimental Section).
Figure 4. NMR spectra of Hf(CH2tBu)4/SiO2-Al2O3-(500): (i) 1H MAS NMR spectrum of Hf(CH2tBu)4/SiO2-Al2O3-(500); (ii) 13C CPMAS solid-state NMR spectrum of Hf(CH2tBu)4/SiO2-Al2O3-(500); (iii) 13C CP-MAS solid-state NMR spectrum of Hf(CH2tBu)4/ SiO2-Al2O3-(500) treated under oxygen.
(Figure 4i), respectively, which are attributed to the protons of Hf(CH2tBu). Note that the peak due to free silanols at 1.8 ppm is absent only with SiO2-(800).10 Solid-State 13C CP-MAS NMR. The spectrum of 1-SiO2-(800) as well as the 13C-labeled surface species Hf(13CH2tBu)4/ SiO2-(800) exhibits two different peaks at 34 and 106 ppm, assigned to Hf(CH2C(CH3)3) and Hf(CH2C(CH3)3), respectively. No resonance for the quaternary carbon, Hf(CH2C(CH3)3), is observed.10 On SiO2-(500), three different peaks, at 34, 95, and 106 ppm, are observed. The peaks at 34 and 106 ppm are attributed respectively to the methyl groups and the secondary carbon of the neopentyl ligand of 1-SiO2-(500). The broad peak at 95 ppm is due to the resonance of the secondary carbon of a bisiloxy complex, (tSiO)2Hf(CH2tBu)2, referred to as 2SiO2-(500). The results of carbon mass balance and the
deconvolution of the two different peaks lead to a ratio 1SiO2-(500)/2-SiO2-(500) of ca. 70%/30%.10 With SiO2-(200), four different peaks, at 34, 83, 95, and 106 ppm, are present (Figure 2ii). Peaks at 34 and 106 ppm are clearly attributed respectively to the chemical shifts of methyl and secondary carbons of the monosiloxy complex 1-SiO2-(200). The amount of 1-SiO2-(200) is not constant, its proportion depending on the initial molar ratio of 1 to surface silanols. The peak at 95 ppm, corresponding to the chemical shift of the secondary carbon of the bisiloxy surface complex 2-SiO2-(200), is always the most intense. The resonance at 83 ppm could be due to one of the following three complexes, either a partially oxidized complex, a trisiloxy surface complex, or another bissiloxy complex in a different local environment. In order to improve the signal-to-noise ratio and to allow certain NMR analyses, the corresponding 13C-enriched
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Scheme 2. Reaction of Hf(CH2tBu)4 with SiO2-(θ) (θ = 800, 500, 200 °C) and with SiO2-Al2O3-(500)
Table 2. Comparison of the Analytical Data Obtained during the Hydrogenolysis, at 150 °C, of 1/SiO2-(θ) (θ = 800, 500, 200 °C) and of 1/SiO2-Al2O3-(500) temperature of oxide pretreatment θ (°C)
methane evolved (mol/mol Hf)
ethane evolved (mol/mol Hf)
methane /ethane
C(equiv) (mol/mol Hf)
C/Hfg before hydrogen
SiO2-(800) SiO2-(500) SiO2-(200) SiO2-Al2O3-(500)
9.3 8.5 5.8 8.4
2.9 2.7 1.9 2.8
3.1 3.1 3.0 3.0
16 ( 2 14 ( 2 10 ( 2 14 ( 2
14 ( 2 12 ( 2 10 ( 2 12 ( 2
complex Hf(13CH2tBu)4/SiO2-(200) 10% randomly enriched on the R-position has been prepared. The 13C-labeled surface species Hf(13CH2tBu)4/SiO2-(200) resulting from the reaction of [Hf(13CH2tBu)4] with SiO2-(200) exhibits very intense signals at 106, 95, and 83 ppm, confirming the attribution of this peak to secondary carbons (Figure 2iii). Treatment under dry oxygen of Hf(13CH2tBu)4/SiO2-(200) leads to drastic changes in the 13C CP-MAS NMR spectrum (Figure 2iv). A single, very sharp peak is observed in the δ(CH2) region; it is at the same chemical shift as, but different in shape and intensity than, in the case of Hf(CH2tBu)4/SiO2-(200). Moreover, the spectrum is also changed in the δ(CH3) and δ(CIV) region. The most intense peak is at 26 ppm and is due to the carbon of the CH3 group; the other at 34 ppm is due to the quaternary carbon. Therefore, when the product is oxidized, even partially, the response of the carbon of the methyl group is much more intense than that of the secondary carbon, and the peaks are very sharp. This is not in agreement with observations in the case of Hf(CH2tBu)4/SiO2-(200). Consequently, in the case of 1/SiO2-(200), these different results indicate the signals at 95 and 83 ppm were both associated with a bis-siloxyhafnium surface, but the Hf center is in a different environment, 20 -SiO2-(500). Since, the infrared and 1H NMR spectra indicate remaining surface
tSi-OH, one may assume that this OH group can play the role of ligand L, in order to stabilize the electron-deficient Hf complex. Consequently, 20 -SiO2-(200) could be described as (tSiO)2(tSiOH)Hf(CH2tBu)2. In conclusion, the reaction of [Hf(CH2tBu)4] with SiO2-(θ), θ = 800, 500, 200 °C, has been investigated. The percentage of hafnium grafted varies from 3.5 ( 0.2% for SiO2-(800) to 5.5 ( 0.2% for SiO2-(500) and 6.2 ( 0.2% for SiO2-(200). All analytical data indicate that only neopentane is evolved during the grafting step. For the three dehydroxylation temperatures, the disappearance of the vibration of isolated silanols at 3747 cm-1 is complete. After grafting of 1 onto SiO2-(500) and SiO2-(200) a broad band between 3740 and 3550 cm-1 corresponding to (tSi-OH) in interaction is still present. The absence of reactivity is probably due to the steric hindrance of the neopentyl groups. Molecular modeling calculations demonstrate that in the case of SiO2-(200) and SiO2-(500), because of the steric hindrance of the neopentyl ligands, not all the silanols groups (3.6 and 1.4 OH nm-2, respectively) can react with the metal center. The maximum density of (tSiO)Hf(CH2tBu)3 and (tSiO)2Hf(CH2tBu)2 is 1.15 and 1.80 mol nm-2, respectively (Figure 3). These values are higher than the experimental ones 0.8 and 1.0 ( 0.2 because the distribution of silanols is
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Figure 5. Infrared spectra of (a) [Hf-H]/SiO2-(800), (b) [Hf-H]/SiO2-(500), (c) [Hf-H]/SiO2-(200), and (d) [Hf-H]/SiO2-Al2O3-(500). (Right) Part the magnification of the ν(Si-H) and ν(Hf-H) region of (a) [Hf-H]/SiO2-(800), (b) [Hf-H]/SiO2-(500), and (c) [Hf-H]/SiO2-(200).
not homogeneous over the silica surface, implying that whatever the dehydroxylation temperature, (tSiO)Hf(CH2tBu)3 is formed. However, there was no experimental evidence supporting the presence of a trisiloxy surface complex [(tSiO)3Hf(CH2tBu)]. In the case of 1/SiO2-Al2O3-(500), the solid-state 13C CPMAS NMR spectrum shows peaks at 26, 34, 72, 95, and 106 ppm (Figure 4ii). The resonances at 106, 95, and 34 are attributed by comparison with SiO2-(500) to 1-SiO2-Al2O3-(500) and 2-SiO2-Al2O3-(500). The peak at 26 ppm could be due to a partial oxidation of surface complexes (vide supra). When Hf(CH2tBu)4/SiO2-Al2O3-(500) is treated under oxygen, the peak at 26 ppm increased and all peaks at 106, 95, and 72 ppm in the δCH2 region disappeared to give a single peak at 82 ppm (Figure 4iii). Therefore, the peak at 72 ppm can be attributed
to a neopentyl hafnium surface complex, as elemental and gas analyses, similar to those of SiO2-(500), are consistent with the presence of a mixture of mono- and bisiloxy complexes. The broad signal at 72 ppm could be attributed to the carbons of some neopentyl groups in interaction with a Lewis functionality of the surface. The broadening is probably due to interaction with (or proximity to) the quadrupolar nucleus of aluminum atoms. Such an interaction has already been reported in the case of the reaction of (η5C5H5)2ZrMe218 and of CH3ReO3 on silica-alumina.32 In the case of SiO2-Al2O3-(500) and Al2O3-(500) this interaction induces a strong upfield shift of (32) Moses, A. W.; Raab, C.; Nelson, R. C.; Leifeste, H. R.; Ramsahye, N. A.; Chattopadhyay, S.; Eckert, J.; Chmelka, B. F.; Scott, S. L. J. Am. Chem. Soc. 2007, 129, 8912–8920.
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Table 3. Chemical Shift δ and 1H DQ MAS Spectral Data (correlation ω1 dimension) of Column 1 [Hf-H]/SiO2-(800), Column 2 [Hf-H]/ SiO2-(500), and Column 3 [Hf-H]/SiO2-Al2O3-(500)a SiO2-(500) δ ( 0.5 ppm
SiO2-(800) δ ( 0.5 ppm [(tSiO)Hf(CH2tBu)(H)2], 4 correlation ω1 dimension [(tSiO)2Hf(H)2], 5 correlation ω1 dimension [(tSiO)3Hf(H)], 6 correlation ω1 dimension
a
SiO2Al2O3-(500) δ ( 0.5 ppm
32 64 (bis-hydride)
no
no
25 and 26* 51 (bis-hydride) *29.5 ppm with peak at 4.5 ppm δ(SiH) of [(tSiO)3SiH] 19* *24.5 ppm with peak at 5.5 ppm, δ(SiH) of [(tSiO)2SiH2].
25 and 26 51(bis-hydride)
25 and 26* 52 ppm (bis-hydride) *28 ppm with peak at 1.7 ppm [δ (tAlOH)] or [δ (tSiOH)] 19* and 22f *23 ppm with peak at 4 ppm, δ(SiH) of [(tSiO)3SiH] f 23.5 ppm with peak at 1.5 ppm [δ (tAlOH)] or [δ (tSiOH)]
19* *23 ppm with peak at 4.0 ppm δ(SiH) of [(tSiO)3SiH]
* and f peaks showing a correlation in ω1 dimension.
the resonance value of the δ13C chemical shift of the Zr-Me resonance.18 For some surface species, there are apparently Lewis acidic centers sufficiently close to interact with a Hf(CH2tBu) group, thus inducing a positive (partial or total) charge on the hafnium atom. To conclude, only with the SiO2-(800) is a unique surface complex obtained: 1-SiO2-(800), (tSiO)Hf(CH2tBu)3. In the case of SiO2-(500) two surface species, 1-SiO2-(500) and 2-SiO2-(500), are formed (∼70%:30%). With SiO2-(200), there is mainly formation of the bis-siloxy surface complex (until 90%) in a different environment, (tSiO)2Hf(CH2tBu)2 (2-SiO2-(200)) and (tSiO)2(tSiOX)Hf(CH2tBu)2 (20 -SiO2-(200)). Finally with SiO2Al2O3-(500), three surface complexes are formed, two neutral, 1-SiO2-Al2O3-(500) and 2-SiO2-Al2O3-(500), and a partially cationic one, [(tSiO)2Hfδþ(CH2tBu)μ(CH2tBu)(surface)δ-], 3, Scheme 2. B. Reaction of 1/SiO2-(θ) (θ = 800, 500, 200 °C) and of 1/ SiO2-Al2O3-(500) with Dihydrogen: Formation and Characterization of Various Surface Hafnium Hydrides. The reaction of [(tSiO)Hf(CH2tBu)3], 1-SiO2-(800), with dihydrogen at different temperatures affords several surface hydrides with a maximum amount of hafnium hydrides when the hydrogenolysis reaction occurs at 150 °C.23 Consequently, the hydrogenolysis reaction of 1/SiO2-(θ) and of 1/SiO2Al2O3-(500) has been carried out at 150 °C. Analytical Results. The hydrogenolysis at 150 °C of 1/ SiO2-(θ) (θ = 800, 500, 200 °C) and of 1/SiO2-Al2O3-(500) leads to surface species referred to as [Hf-H]/SiO2-(θ) and [Hf-H]/ SiO2-Al2O3-(500), respectively. In the gas phase only methane and ethane in a ratio methane/ethane = 3.1 ( 0.1 are detected. The mass balance with respect to the number of carbon atoms, calculated from GC results (Table 2, column 5), is consistent with the number of carbon atoms found by elemental analysis before hydrogenolysis (Table 1, column 6). Moreover, the elemental analysis of the residual solid indicates that little carbon remains, indicating that the quasi-totality of Hf-CH2tBu bonds were cleaved under H2, at 150 °C. In Situ IR Spectroscopy. The silica pellets of 1/SiO2-(θ) (θ = 800, 500, 200 °C) and of 1/SiO2-Al2O3-(500) are heated at 150 °C for 17 h under 500 Torr of dry hydrogen (Figure 5). In all cases, IR spectra show the almost complete disappearance of the ν(CH) (3000-2500 cm-1) and δ(CH) (1800-1300 cm-1) bands (97.5% of the intensity of the ν(CH) bands is lost). There is a concomitant appearance of new vibrations bands centered at 1700 cm-1, exchangeable under D2 and assigned
Figure 6. 1H solid-state NMR spectra of (a) [Hf-H]/SiO2-(800), (b) [Hf-H]/SiO2-(500), and (c) [Hf-H]/SiO2-(200), *Spinning band.
to ν(Hf-H).23 Two broad bands appear centered at ca. 2260 and 2200 cm-1, not exchangeable under D2, attributed to ν(SiH) and ν(SiH2), respectively.16,23,33-35 Moreover, the intensity ratio of the peaks assigned is dependent on θ. In the case of 1-SiO2-(800), the pattern of bands ν(Hf-H) centered at 1700 cm-1 has been assigned to the three main surface hafnium hydrides: [(tSiO)Hf(CH2tBu)(H)2], 4 [1651, 1685 cm-1], [(tSiO)2Hf(H)2], 5 [1675, 1720 cm-1], [(tSiO)3Hf(H)], 6 [1700 cm-1], and [(tSiO)2Si(H)2] [(tSiO)3SiH] (Figure 5a).23 Compared to that of 1-SiO2-(800), in the IR spectra of [Hf-H]/SiO2-(500), [Hf-H]/SiO2-(200), and [Hf-H]/ SiO2-Al2O3-(500) the pattern corresponding to ν(Hf-H) vibrations is mainly centered at 1702 cm-1 with a weak shoulder at (33) Campbell-Ferguson, H. J.; Ebsworth, E. A. V.; MacDiarmid, A. G.; Yoshioka, T. J. Phys. Chem. 1967, 71, 723–6. (34) Silverstein, R. M.; Bassler, G. C.; Morill, T. C. Spectrometric Identification of Organic Compounds, 4th ed.; Wiley: New York, 1992. (35) Rataboul, F.; Baudouin, A.; Thieuleux, C.; Veyre, L.; Coperet, C.; Thivolle-Cazat, J.; Basset, J.-M.; Lesage, A.; Emsley, L. J. Am. Chem. Soc. 2004, 126, 12541–12550.
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Scheme 3. Description of the Possible Hydride Surface Complexes on SiO2-(θ) (θ = 800, 500 °C) and SiO2-Al2O3-(500)
1675 cm-1, indicating that there was mainly formation of [(tSiO)3Hf(H)], 6, (Figure 5b-d). The IR spectrum of [Hf-H]/SiO2-Al2O3-(500), besides the presence of [Al-H] bonds (ν(AlH) = 1935 cm-1), is similar to that of [Hf-H]/SiO2-(500), indicating there is mainly formation of 5, 6, [(tSiO)3SiH], and [(tSiO)2SiH2]. The band ν(AlH) at 1935 cm-1 is found to be in the upper range of ν(Al-H) absorption reported in the case of Zr hydride/Al2O3 (in this case the vibration of the ν(Al-H) is equal to 1910 cm-1).36 Solid-State NMR Spectroscopy. The 1H solid-state NMR spectra of [Hf-H]/SiO2-(θ) show signals between 19 and 32 ppm. Their presence, number, and intensity vary with θ, Figure 6. The two broad bands at 25 and 19 ppm have previously been assigned to [(tSiO)2Hf(H)2], 5, and [(tSiO)3Hf(H)], 6, respectively, and the peak at 32 ppm to [(tSiO)Hf(CH2tBu)(H)2], 4, which is observed only with [Hf-H]/SiO2-(800) (Figure 6a).23 In the 1H MAS spectrum of [Hf-H]/SiO2Al2O3-(500) the signals at 25 and 19 ppm confirm the formation of surface hydrides 5 and 6 (Figure 6b) as in the case of [Hf-H]/SiO2-(200), yet an additional peak centered at 22 ppm is observed and could be assigned to 6 in a different local environment (Figure 6c). Double-Quantum (DQ) Proton Spectroscopy. To further assign these various signals, double-quantum (DQ) proton spectroscopy under magic angle spinning (MAS) has been performed on [Hf-H]/SiO2-(θ) and [Hf-H]/SiO2Al2O3-(500). Multiquantum proton spectroscopies under fast MAS have been successfully applied to the characterization of hydrogen-bonding structures, π-π packing arrangements, and defect sites in zeolites,37 and used for the characterization of surface complexes.23,35,38,39 From numerous decoupling sequences, we have chosen the homonuclear (36) Joubert, J.; Delbecq, F.; Sautet, P.; Le Roux, E.; Taoufik, M.; Thieuleux, C.; Blanc, F.; Coperet, C.; Thivolle-Cazat, J.; Basset, J. M. J. Am. Chem. Soc. 2006, 128, 9157–9169. (37) Brown, S. P. S.; H., W. Chem. Rev. 2001, 101, 4125. (38) Avenier, P.; Lesage, A.; Taoufik, M.; Baudouin, A.; De Mallmann, A.; Fiddy, S.; Vautier, M.; Veyre, L.; Basset, J.-M.; Emsley, L.; Quadrelli, E. A. J. Am. Chem. Soc. 2007, 129, 176–186. (39) Gauvin, R. M.; Delevoye, L.; Hassan, R. A.; Keldenich, J.; Mortreux, A. Inorg. Chem. 2007, 46, 1062–1070.
symmetry-based post-C7 experiment introduced by Levitt et al.,40 which is robust with respect to chemical shift offset and rf inhomogeneity41 and has been extensively used with good efficiency on numerous nuclei including the proton.42 In this experiment, correlations are observed between pairs of dipolar-coupled protons. The DQ frequency in the ω1 dimension corresponds to the sum of the two single quantum frequencies of the two coupled protons and correlates in the ω2 dimension with the two corresponding proton resonances. The observation of a DQ peak implies a close proximity between the two protons involved in this correlation. The 1H DQ MAS spectra of [Hf-H]/SiO2-(800) and [Hf-H]/SiO2-(500) have already been reported.23 The reported results confirm the presence in the two samples of the bishydride hafnium [(tSiO)2Hf(H)2], 5, through the correlation between the peaks at 25 and 26 ppm and the peaks at 51 ppm in the ω1 dimension. Conversely to [Hf-H]/SiO2-(800), in [Hf-H]/SiO2-(500) no correlation band between these two proton peaks and the resonance of SiH of [(tSiO)3SiH] at 4.5 ( 0.5 ppm is observed. Consequently, [(tSiO)2Hf(H)2] on [Hf-H]/SiO2-(500) is in a different local environment than 5 and is referred as to 50 . In the same way, the proton resonance at 19 ppm corresponding to [(tSiO)3Hf(H)], 6, is in [Hf-H]/SiO2-(800) only correlated with the SiH at 4.0 ( 0.5 ppm resonance of [(tSiO)2SiH2]. In [Hf-H]/SiO2-(500), [(tSiO)3Hf(H)], a correlation between the proton resonance at 19 ppm and the SiH resonance of [(tSiO)3SiH] (23 ppm) in the ω1 dimension has been evidenced. The monohydride hafnium on [Hf-H]/ SiO2-(500) is in a different local environment than 6 and referred to as 60 (Table 3 and Scheme 3). The 1H DQ MAS spectra of [Hf-H]/SiO2Al2O3-(500) (Figure 7), confirms the presence of the bis-hydride hafnium (resonances at 25 and 26 ppm), which correlates to a peak (40) Hohwy, M.; Jakobsen, H. J.; Eden, M.; Levitt, M. H.; Nielsen, N. C. J. Chem. Phys. 1998, 108, 2686–2694. (41) Karlsson, T.; Popham, J. M.; Long, J. R.; Oyler, N.; Drobny, G. P. J. Am. Chem. Soc. 2003, 125, 7394–7407. (42) Brown, S. P.; Lesage, A.; Elena, B.; Emsley, L. J. Am. Chem. Soc. 2004, 126, 13230–13231.
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Figure 7. 1H DQ MAS spectrum recorded for 1/SiO2Al2O3-(500) after 17 h treatment at 150 °C under 500 Torr H2 (21pC7 blocks, 400 increments, 16 scans, RD = 16 s, 18 kHz probe 3.2 mm).
centered at 1.7 ppm. This corresponds to the protons of SiO2Al2O3-(500) (tSiOH and tAlOH). Consequently, the bis-hydride hafnium on [Hf-H]/SiO2-Al2O3-(500) is either close to the tSiOH group, referred to as 500 , or close to the tAlOH group and is therefore in another environment referred to as 5000 (Scheme 3). The two resonances at 19 and 22 ppm do not present autocorrelation in the ω1 dimension, indicating that there are two sorts of monohydride hafnium, [(tSiO)3Hf(H)]. The peak at 19 ppm presents both interactions with the peak at 1.5 ppm, the resonance of tAl-OH or tSi-OH groups, and with the peak at 4 ppm, the resonance of [(tSiO)3SiH]. The local environment is probably similar to 60 . The peak at 22 ppm in interaction with a proton resonance at 0 ppm is close to the proton of tAl-OH and is referred to as 600 (Table 3 and Figure 7). In conclusion, 1/SiO2-(θ) and 1/SiO2-Al2O3-(500) under hydrogen at 150 °C led to the same surface mono- and bishydrides but in different local environments. Scheme 3 depicts all the surface hydrides formed on these two oxides. The relative proportion of these surface hydrides is not always the same. Experimental results clearly show that the hafnium bis-hydride [(tSiO)2Hf(H)2], 5, is the major product on SiO2-(800), SiO2-(500), and SiO2-Al2O3-(500). Conversely, on SiO2-(200) the monohydride is the most abundant. Nevertheless, these hydrides are in different local environments, and taking into account the presence of remaining hydroxyl surface groups, we have given a rational interpretation of the corresponding structures, namely for the [(tSiO)2Hf(H)2], 5, 50 , and 500 , and for [(tSiO)3Hf(H)], 6, 60 , and 600 . Only on SiO2-(800) do the hafnium hydrides not have spatially close [tSi-OH] functionalities.
Conclusion [Hf(CH2tBu)4], 1, reacts with SiO2-(θ) (θ = 800, 500, 200 °C) and with SiO2-Al2O3-(500) to give a mixture of mono- and bissiloxy surface neopentyl hafnium complexes [(tSiO)xHf(CH2tBu)4-x] (x=1, 2) in different local arrangements. Under hydrogen these complexes afford [(tSiO)yHf(H)4-y] (y=2, 3)
in different ratios and local environments. All surface complexes have been quantitatively and structurally characterized by elemental analysis, labeling, infrared, 1H/13C solid-state NMR, and 1H DQ solid-state NMR. This work highlights the different possible structures of the active site. We will see in other work that surface hafnium hydrides on different supports (alumina, silica-alumina, and silica) exhibit very drastic differences in activity in the polymerization of alpha olefins. We expect that these variations in activity are related to the multiplicity of sites depending on the support.
Experimental Part All experiments were carried out by using standard air-free methodology in an argon-filled Vacuum Atmospheres glovebox, on a Schlenk line, or in a Schlenk-type apparatus interfaced to a high-vacuum line (10-5 Torr). Pentane, hexane, THF, and ether were purified according to published procedures,43 stored under argon over 3 A˚ molecular sieves, and degassed prior to use. C6D6 (SDS) was distilled over Na/benzophenone. HfCl4 (Cezus, 270 ppm Zr) was used as received. The synthesis of Hf(CH2tBu)4] and the thermal treatment of the oxide support silica Aerosil from Degussa and the silica-alumina HA-S-HPV from Akzo-Nobel have already been reported elsewhere.10 Hydrogen was dried over a deoxo catalyst (BASF R3-11 þ 4 A˚ molecular sieves) prior to use. Oxygen was dried over 4 A˚ molecular sieves. Gas phase analyses were performed on a Hewlett-Packard 5890 series II gas chromatograph equipped with a flame ionization detector and a KCl/Al2O3 on a fused silica column (50 0.32 mm). Elemental analyses were performed at the CNRS Central Analysis Department of Solaize or at the LSEO of Dijon. Infrared spectra were recorded on a Nicolet 550-FT spectrometer by using a custom infrared cell equipped with CaF2 windows. Typically, 16 scans were accumulated for each spectrum (resolution 2 cm-1). The 1H MAS and 13C CP-MAS NMR spectra were recorded on a Bruker DSX-300 or a Bruker Avance 500 spectrometer equipped with a standard 4 mm double-bearing probehead. Samples were introduced under argon in a zirconia rotor, which was then tightly closed. The spinning rate was typically 10kHz. (43) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed.; 1988.
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Typical cross-polarization sequences were used, with 5 ms contact time and a recycle delay of 1 to 4 s, to allow the complete relaxation of the 1H nuclei. All chemical shifts are given with the respect to TMS, as an external reference. Double-Quantum Spectroscopy. The proton DQ experiments were designed as follows: rf field strength was 70 kHz (7*ωR) during the excitation and reconversion periods, which were chosen equal to 200 μs (corresponding to 7 post C7 basic elements); 512 increments of 32 scans each were collected with a 4 s recycle delay, which gave a total experiment time of 18 h. Processing was done with a 50 Hz line broadening in both dimensions and one zero filling in the ω1 dimension. Modeling Molecular Calculations. To check if the experimental weight percentage of hafnium corresponded to the highest loading achievable on the surface, a molecular modeling of 1 was performed by using the Sybyl computer modeling program. The molecular fragments Hf(CH2tBu)3 and Hf(CH2tBu)2 were attached to a silanol group of a modeled silica particle SiO2-(500) by replacing the hydrogen atom with [tSiO] and minimized by using the molecular mechanics Tripos force field.44-46 Preparation of Surface Complexes. Two different workups were used in order to graft Hf(CH2tBu)4, 1, onto SiO2-(θ) and SiO2-Al2O3-(500): either impregnation in a solvent (pentane) at room temperature or mechanical mixing at 70 °C, in each case for two hours. The results of elemental analysis and (44) Clark, M.; Cramer, R. D., III.; Van Opdenbosch, N. J. Comput. Chem. 1989, 10, 982–1012. (45) Steward, J. J. P. J. Comput. Chem. 1991, 10, 320. (46) Connolly, M. L. Science 1983, 221, 709–713.
Tosin et al. quantification of the gas evolved during the grafting and subsequent hydrogenolysis are given in Table 1. General Procedure. A mixture of [Hf(CH2tBu)4], 1 (135 mg, 290 μmol, 1.3 equiv), and SiO2-(500) (0.5 g, 230 μmol OH) was stirred at 70 °C for two hours. All volatile compounds were condensed into another reactor (of known volume) so as to quantify neopentane evolved during the grafting. Pentane (10 mL) was introduced into the reactor by distillation, and the solid was washed three times. The resulting white powder was dried under vacuum (10-5 Torr) to yield 0.5 g of Hf(CH2tBu)4/SiO2-(500). Gas analyses by chromatography indicate the formation of 197 ( 20 μmol of neopentane during the grafting (1.3 ( 0.1 CH3tBu/Hf, 0.6 ( 0.1 CH3tBu/(tSiOH), 0.6 ( 0.1 Hf/(tSiOH)). Elemental analyses of Hf(CH2tBu)4/ SiO2-(500): Hf 5.0 wt %, C 4.1 wt % (12 ( 2 C/Hf). Solid-state MAS 1H NMR (300 MHz): δ 0.8 ppm. CP/MAS 13C NMR: δ 106, 95, and 34 ppm. Treatment under H2. The solid was introduced in a standard break-seal glass apparatus (of known volume) under strict exclusion of air. It was heated at 150 °C in the presence of a large excess of H2 (500 mbar). H2 was dried over molecular sieves and deoxo catalyst prior to use. After 17 h, the gaseous products were quantified by gas chromatography, and the solid was analyzed by NMR solid-state spectroscopy. Treatment under Dry Oxygen. The solid was introduced in a batch reactor of known volume. After evacuation of argon, a large excess of oxygen (500 mbar) was added. Oxygen was dried over molecular sieves prior to use. The reaction was monitored by IR, 1H MAS, and 13C CP-MAS NMR spectroscopies.